The DØ detector is located at the Fermilab Tevatron [39,40,41,42], presently the world's highest-energy hadron collider, with a center-of-mass energy of 1800 GeV. A schematic of the accelerator complex is shown in Figure 3.1.
Figure 3.1: Schematic of the Fermilab accelerator complex (not to scale).
[43, p. 112]
The Tevatron is a proton storage ring, composed of superconducting magnets. The ring is filled with bunches of protons and antiprotons, which circulate in opposite directions. At the B0 and D0 experimental areas, these beams are made to collide with each other. The process of filling the ring is quite complicated; a summary of the major steps is given below, but the reader should consult the cited references for more details.
The beams originate in the preaccelerator. There, 
ions are
formed and accelerated to 750 keV by an electrostatic Cockroft-Walton
accelerator. The preaccelerator operates in a pulsed mode with a
frequency of 15 Hz. The ions are bunched and transported to the start
of the Linac. The Linac is a 150 m long linear accelerator, which
boosts the energy of the ions to 200 MeV
.
After emerging from the
Linac, the ions are passed through a carbon foil which strips off
the electrons, leaving bare protons. The protons are then
injected into the Booster, a 151 m diameter synchrotron.
(A synchrotron is a device which confines charged particles in a closed
orbit using bending magnets. RF cavities can be used to increase the
energy of the stored particles; when this is done, the field of the
bending magnets must also be increased in a synchronous manner in order
to keep the particles in the same orbit.)
One of the interesting features of the Booster is its rapid cycle rate
of 15 Hz. To achieve this, the
magnets are combined with capacitor banks to form LC circuits which
resonate at 15 Hz. The Booster accelerates the protons to an energy
of 8 GeV. The protons are then injected into the Main Ring, a large
(1000 m radius) synchrotron composed of conventional magnets. The
Main Ring lies mostly in a plane, except at the B0 and D0 experimental areas
where it is bent into overpasses to allow room for the detectors
(the separation between the Main Ring and the Tevatron is 19 feet at B0
and 89.2 inches at D0). Protons in the Main Ring can be used to
make antiprotons (see below), or they can be accelerated to 150 GeV
and injected into the Tevatron.
The Tevatron is a proton synchrotron made from superconducting magnets [40,42]. It lies just below the Main Ring in the accelerator tunnel, and has a maximum beam energy of 900 GeV. (Upgrades to the cryogenic system are expected to raise this to 1000 GeV.) The Tevatron can be operated in one of two major modes. In fixed-target mode, the Tevatron is filled with protons which are accelerated and then extracted and directed towards numerous experimental areas. This cycle repeats with a frequency of about once per minute. In collider mode, the Tevatron is filled with six bunches of protons and six bunches of antiprotons, traveling in opposite directions. The beams are accelerated to the maximum energy of 900 GeV each and allowed to collide at the B0 and D0 experimental areas. (At other points where the beams would collide, they are kept apart by electrostatic separators). The beams are typically kept colliding for about 20 hours, after which the machine is emptied and refilled with new batches of protons and antiprotons.
The remaining major part of the accelerator complex is the antiproton
source [41,44],
which is used to produce and store antiprotons for use in the
collider.
While collisions are occurring in the Tevatron, the Main Ring
continually runs antiproton production cycles at a rate of one
every 2.4 s. Protons are accelerated to 120 GeV and extracted onto
a nickel target. Each of these collisions produces a spray of nuclear
debris, which includes some antiprotons.
Immediately following the target is a lithium lens,
a cylindrical piece of lithium through which a large (
)
current is passed. This generates an azimuthal magnetic field which
acts to focus negatively-charged particles passing through it.
Following the lens is a bending magnet which selects
negatively-charged particles
with energies of 8 GeV and transports them to the Debuncher. The
Debuncher is a storage ring in which antiprotons are first `debunched'
(rotated in phase space from a configuration with a small time spread
and large momentum spread to one with a large time spread but small
momentum spread) and then stochastically `cooled' to further reduce the
momentum spread. Stochastic cooling [44,45]
operates by measuring the
trajectory of collections of particles relative to the desired orbit.
From this information, a correction signal is derived
which is passed across the ring to kicker
electrodes which apply a force on the particles to move them back towards
the desired orbit. The effect on any
single particle is very small due to the incoherent contribution of all
the other particles near it in the beam, but when repeated over a large
number of turns, the effect becomes significant. The antiprotons are
kept in the Debuncher until just before the next pulse arrives,
about 2.4 s later.
They are then transferred to the Accumulator, another storage ring which
lies inside the Debuncher. There, cooling continues for several
hours, and eventually the antiprotons settle into a dense core near the
inner radius of the Accumulator. When enough have accumulated to fill
the Tevatron (typically on the order of 50 -- 
), they
are extracted from the Accumulator, accelerated to 150 GeV in the Main
Ring, and injected in bunches into the Tevatron.
Table 3.1: Run 1A Tevatron Parameters. [46, p. 17]
[47, ch. 2]
[43, app. A]
Some of the major parameters of the Tevatron for run 1A are given in
Table 3.1. A more detailed introduction to the accelerator may
be found in [43, appendix A].